An intensive study on the optical, rheological, and electrokinetic properties of polyvinyl alcohol-capped nanogold
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Low-temperature-assisted wet chemical synthesis of nanogold (NG) using gold hydroxide, a new precursor salt in the presence of a macroscopic ligand poly(vinyl alcohol) PVA in water in the form of nanofluid, is reported for the first time in this article. In the absorption spectra, the surface Plasmon resonance absorption band in the range of 520–545 nm signifies the formation of NG via a controlled Au3+ + 3e → Au reaction grafted in small assemblies with polymer. Absorption maximum increases nonlinearly with Au-contents up to 100 µM Au in Au-PVA charge-transfer complex. Marked enhancement in the peak intensity of some of the vibration bands of PVA polymer such as C–H stretching, C=O stretching, CH2 bending, and C–C in-plane bending in the presence of NG reveals an interfacial interaction between NG and oxidized PVA via C=O group. Execution of shear thinning behavior regardless of the Au-content strongly suggests that crosslinking exists between NG and PVA in Au-PVA rheo-optical nanofluids. Hydrodynamic diameter and polydispersity index draw a nonlinear path with the Au doping with 30.0 g/L PVA in water over a wide region of 5–100 μM Au covered in this study. Enhancement in the zetapotential of Au-PVA nanofluid over bare PVA in water is ascribed to buildup of nonbonding electrons of “–C=O” moieties from the oxidized PVA on the NG surface. Displaying of lattice fringes in the microscopic image of core–shell Au-PVA nanostructure confirms that crystalline nature of NG core with inter planar spacing 0.235 nm corresponds to Au (111) plane.
KeywordsNanofluids Dynamic light scattering Electrokinetic effect Interfacial interaction Core–shell nanostructure
Nanofluids (NFs) represent a relatively new class of functional and engineered materials which consist of nanoscale materials (2–50 nm size) such as metals (Cu, Ag, Au, Pt, etc.), oxides (Al2O3, SiO2, Fe3O4, TiO2, etc.), carbides (SiC, WC, etc.), carbon (fullerenes, graphene, and carbon nanotubes) dispersed in water, mineral oils, ethylene glycol, polymer solutions, and biofluids imperative for diverse applications of medicines, color pigments, biological sensors, energy transfer, coolants, and many others [1, 2, 3, 4, 5, 6, 7]. The innovative idea of the NFs was first introduced by Dr. Choi of Argonne National Laboratory as early as 1995 .
Soon after the synthesis of nanogold (NG) by Michael Faraday in 1857 , NFs containing noble metal nanoparticles (NPs) have been attracting extensive attention in scientific communities on account of potential applications of NG in the various fields such as sensing [9, 10, 11, 12], biomedicals [9, 10, 11, 12, 13], catalysis [9, 10, 11, 12], photonics [11, 12, 13], memory devices [11, 12, 13], and heat transfer [14, 15]. The synthesis of metal-polymer NFs with controlled microstructure and functional properties is an integral part of an emerging and rapidly growing “materials technology”. The efforts are being made on possibility to control over the microstructures via innovative synthetic methods. In this regard, various bottom-up approaches, such as photolysis [10, 13], chemical reduction [12, 16], and template methods [17, 18], have been adopted for the synthesis of NG of controlled shapes and sizes. Moreover, in an in situ technique, a nanostructured metal is produced within a polymer either via a chemical decomposition of a metal complex or by a chemical reduction reaction of metal salts by using a proper reducing agent in a solvent. In particular, NG of various architectures such as rods, plates, disks, cubes, spheres, stars, etc. can be synthesized by proper manipulation of experimental parameters in the presence of a macroscopic shape-regulating agents in the form of polymer [9, 10, 11, 12, 13, 16].
Commonly used polymers for surface stabilizing noble metals like gold in a nanostructure in a liquid medium include poly(vinyl pyrrolidone) PVP [13, 19, 20, 21, 22, 23, 24], poly(ethylene glycol) PEG , poly(vinyl alcohol) (PVA) [9, 11, 12, 16], polymethyl methacrylate , and poly(vinylidene fluoride) [25, 26]. Colloidal stability in NFs can be achieved by surrounding particles (1) with an electrical double-layer (i.e., an electrostatic stabilization), (2) by adsorbing or chemically attaching polymeric molecules (i.e., a steric stabilization), or (3) by a free polymer in the dispersion medium (i.e., depletion stabilization) [5, 6, 27, 28]. Combination of the first two stabilization mechanisms leads to electrosteric stabilization. So far as the electrostatic stabilization is concern, particles with zetapotential (ξ) ≥ (±)30 mV are normally considered as stable nanofluid [27, 28].
Herein, we discuss a simple in situ synthesis method to develop NG from gold hydroxide, a relatively new precursor salt [20, 21, 22, 23, 24], in the presence of a macroscopic stabilizer and reducer like polyvinyl alcohol (PVA) in an aqueous medium. To the best of our knowledge, this is the first report on synthesis of NG using gold hydroxide as precursor salt in an aqueous medium at relatively low reaction temperature of 40 °C. Porel et al.  have synthesized polygonal Au nanoplates using HAuCl4 salt in the presence of PVA by a thermal treatment at 100–170 °C. In an another experiment, Tripathy et al.  reported synthesis of nanoplates of Au of various architectures using HAuCl4 salt in the presence of PVA via an in situ wet chemical route at 50–60 °C. As our salt gives free Au3+ ions in solution whose reduction potential is less (i.e., Au3+ + 3e → Au, E 0 = 0.8 V) as compared to widely used HAuCl4 salt (i.e., AuCl4 − + 3e → Au + 4Cl−, E 0 = 1.1 V), reduction from Au3+ ions to Au atom is easier and hence reduction reaction occurs at a low temperature. The samples were characterized using UV–Visible, parallel plate rotational rheometer, zetapotential, dynamic light scattering (DLS), and high-resolution transmission electron microscope (HRTEM).
Reagent grade Au(OH)3 powder was purchased from Alfa Aesar and was dissolved in dilute HNO3 to prepare a stock solution of 2 mM Au(NO3)3. Polyvinyl alcohol (PVA) of average molecular weight ~125,000 was purchased from Sigma Aldrich. All the chemicals were used as received without further purification.
Synthesis of PVA-Au NFs in water
At first, an aqueous solution of PVA (30 g/L) was prepared in water by magnetic stirring for 3 h at 60–70 °C. After this, a specific volume of gold nitrate solution was added drop wise while stirring to a 5 mL of 30 g/L PVA solution in water at 40 °C. Light purple-colored NFs were obtained after magnetic stirring the solution for 5 min and then sonicated at 40 °C in an ultrasonicator (OSCAR, 20 kHz frequency and 250 W power) for 5 min. PVA-Au NFs thus obtained were studied using experimental techniques such as UV–Visible, Fourier transform infrared (FTIR) spectrometer, rheometer, DLS, zeta potential, and HRTEM.
The optical absorption spectra of aqueous 30 g/L PVA solution with and without NG were measured under identical conditions on a Perkin–Elmer double beam spectrophotometer (LAMBDA 1050). The sample was filled in a transparent cell of quartz (10-mm optical length) and the spectrum was recorded against a reference (water, or 30 g/L PVA with water) in an identical cell. Vibration spectra of the solution have been studied with a Thermo Nicolet Corporation FTIR spectrometer (Model NEXUS-870). The spectra have been recorded in an attenuated total reflectance mode using a ZnSe crystal as a sample holder. Zetapotential (ξ), hydrodynamic diameter (L hd), and polydispersity index (PDI) were measured by using a Malvern Nano ZS instrument using phase analysis light scattering technique. Diluted samples were sonicated for 60 s prior to measurements. The samples were analyzed three times at 25 °C. The rheological properties of Au-PVA NFs of varied compositions in water were measured using a rotational rheometer (TA instruments, model: AR-1000) of parallel plate geometry, with an upper plate of diameter 40 mm. To measure the data, a few drops of the fluid were put on the lower plate of the system with a torque varying from 0.2 to 0.5 μN-m at shear rates γ varying from 10 to 500 s−1. The shear viscosity and shear stress were measured at selective γ values in the 10–500 s−1 range. Microscopic image of PVA-capped Au was studied with a JEM–2100 (JEOL, Japan) instrument. HRTEM samples were prepared by placing one drop of diluted solution on a carbon coated 400-mesh copper grid and allowing the sample to dry in desiccators at room temperature.
Results and discussion
UV–Visible and FTIR spectra
Rheology in Au-PVA NFs
DLS and Zetapotential in Au-PVA NFs
The values of ξ, L hd, and PDI determined from zeta potential and DLS for bare PVA solution and synthesized Au-PVA NFs in water
ξ value (mV)
L hd value (nm)
We also studied how the ξ value varies in these Au-PVA NFs with the Au-content in an aqueous medium. As depicted in the inset of Fig. 9, ξ value shows peak maxima near 25 μM Au which acquires a relatively small η value with a reasonably decreased PDI index (Table 1) in the Au-PVA NFs. As a result, effectively large ξ value has a correlation to a reasonably low viscous sample. Such a sample lets a light-induced mechanical perturbation propagate through the sample. In the other words, a PVA polymer gets easily adsorbed on an Au nanosurface via –OH and >C=O sites of a PVA polymer of thin surface layer in a core–shell structure. Small ξ value in our samples (i.e., less than −30.0 mV) suggests that colloidal stability is due to both electrostatic and steric stabilization mechanisms. In an experiment, Rahme et al.  found that the ξ value in a nanofluid also depends on the host structure. A ξ value had been decreased from (−)35 mV to (−)1 mV in a gold-reinforced PEG nanofluid in a nonlinear path when PEG molecular weight was increased from 2100 to 51,400 g mol−1. They remarked that such decrease in the ξ value with an increased molecular weight of a liquid carrier is not surprising as zeta potential is measured at the surface plane of the hydrodynamic sphere of diameter and the surface plane of the hydrodynamic sphere is far from the surface of flocculates in a stable nanofluid. PEG-capped Au NPs exhibit an average ξ value as (−)13.9 mV for the highest loading of polymer as a result of steric stabilization compared to a large ξ value of (−)39.5 mV shown in citrate-capped Au NPs .
Surface encapsulation study by microstructure and absorption spectra in Au-PVA NF
Gold-PVA NFs were developed in an aqueous medium via a chemical reduction route at as low temperature as 40 °C using gold nitrate as precursor salt. It has been found that λ max of the SPR band increases as PVA-capped NG cluster size increases with Au-content. FTIR spectra suggest existence of an interfacial interaction between C=O group of oxidized PVA and surface of NG. A scheme was proposed to support amplification in the band intensity of –C=O group in the PVA polymer in presence of NG. Electrokinetic study of the Au-PVA NFs further reveals formation of assemblies between Au and PVA via interaction between C=O group of PVA and NG. From the study, it is found that a strong correlation exists between optical and electrokinetic parameters in the Au-PVA NFs. HRTEM images validate the optical, rheological, and electrokinetic study on the NFs. Development of core–shell nanostructure of Au-PVA NFs in an aqueous medium via a green route could find applications in the field of biosensors.
This work has been supported by the Silicon Institute of Technology, Silicon Hills, Bhubaneswar, India.
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